User Manual

Architecture
Nortel Networks Confidential
3–42
PE/DCL/DD/0063
411–9001–063
Standard 12.07/EN September 2000
GSM/BSS V12
During the call establishment, the BTS computes the timing advance value and
sends it within CHANNEL REQUIRED message to the BSC. If this value is above
the threshold, then the BSC rejects the call establishment.
In ongoing call conditions, the timing advance is calculated at regular intervals and
sent to the MS over the downlink SACCH channel.
The calculation is based on the following:
other measurements taken during demodulation
the timing advance used by the mobile station that is returned in the layer 1
header of the uplink SACCH
Discontinuous transmission (DTX)
Discontinuous transmission allows signals to be sent over the radio channel alone
when a speech signal is present. This limits interference and MS power
consumption. For each call, the MSC indicates whether the BSS “does not use” or
“may use” the DTX.
The principle behind discontinuous transmission is as follows:
The base or mobile vocoder has a Voice Activity Detector (VAD) that detects if the
frame constructed every 20 milliseconds contains speech. If the frame does not
contain speech, the vocoder constructs a special frame called the SIlence Descriptor
(SID) that contains all the background noise description elements. This frame is sent
to produce a comfort noise at the far end, and radio transmission stops.
The vocoder periodically reassesses the ambient noise and reconstructs the SID
frame. The frame produced in this way is sent in step with the SACCH (once every
four 26-frame multiframes (480 milliseconds)).
When the vocoder detects new speech activity, a special SID frame indicating the
End Of Silence (EOS) is sent, and normal speech frame sending resumes.
On the receive end, additional processing sequences interpret the incoming traffic
frame types (speech, SID, FACCH, nothing) using the related flags (BFI, SID, TAF)
and perform the appropriate operations.
The DTX is allowed for data in non-transparent mode.
BCCH filling
The BCCH frequency must be transmitted continuously so mobile stations can
perform field strength measurements in neighbouring cells.
Continuous transmission is accomplished in the following ways:
When frequency hopping is not used, the TRX uses the BCCH frequency as the
carrier frequency for all the channels it supports. The TRX sends fillers on the
BCCH frequency although it may have nothing to send in a given time slot.